Wireless communication systems are generally allocated a portion of the radio frequency (RF) spectrum for their operation. The allocated portion of the spectrum is divided into communication channels and channels are distinguished by frequency, time or code assignments, or by some combination of these assignments. Each of these communication channels will be referred to as conventional channels, and a conventional channel will correspond to a fill-duplex channel unless otherwise noted. The establishment of a communication link in a communication system depends not only on the availability of a conventional channel but also on the quality of communication that will result from the use of a given available conventional channel.
In wireless communication systems, a conventional channel is used for communication between a base station and a subscriber station. A base station provides coverage to a geographic area known as a cell and may be a point-of presence providing connection between the subscriber station and a wide area network such as a Public Switched Telephone Network (PSTN). The underlying motivation for the use of cells in wireless systems is the reuse of the RF spectrum in geographically different areas. The reuse of the frequency spectrum can introduce co-channel (intercell) interference between users in different cells that share a common conventional channel. If co-channel interference is not carefully controlled, it can severely degrade the quality of communications. System capacity is in general limited by interference because of the reduction in the number of reusable channels of acceptable quality.
Another source of conventional channel quality degradation is adjacent channel (intracell) interference caused by other conventional channels within a given cell. Ideally, within a given cell each conventional channel should be completely isolated from all of the other conventional channels (orthogonal). In practical systems, full orthogonality between channels can not be ensured because of the complexity and cost such a requirement would place on the system design. For example, adjacent channel interference can result, in FDMA systems, from RF carrier frequency offsets and imperfect filters; in TDMA systems, from timing offset and jitter; and, in CDMA systems, from synchronization inaccuracies or RF multipath propagation.
The more recently introduced SDMA systems (Roy et al., U.S. Pat. No. 5,515,378) allow multiple subscribers within a given cell to simultaneously share the same conventional channel without interfering with one another, and further, allow more frequent reuse of conventional channels within a geographical area covering many cells. SDMA exploits the spatial distribution of subscribers in order to increase usable system capacity. Because subscribers tend to be distributed over a cell area, each subscriber-base station pair will tend to have a unique spatial signature characterizing how the base station antenna array receives signals from the subscriber station, and a second spatial signature characterizing how the base station antenna array transmits signals to the subscriber station. Subscribers sharing the same conventional channel are said to be using different spatial channels. As in the case of FDMA, TDMA, and CDMA systems, spatial channels in a SDMA system may not be perfectly orthogonal because of hardware limitations and multipath propagation. It should be noted that non-spatial multiplexing (e.g., FDMA, TDMA, and CDMA), when used in combination with antenna array patterns that are controlled by using spatial processing, is referred to as SDMA in the context of this invention. In practice, spatial signatures and antenna arrays can be used in a non-spatial-division-multiple-access system configuration for enhancing communications between base stations and subscribers by use of spatial signal processing techniques. In these cases, the label SDMA will still be used in the context of the description that follows.
FIG. 1 shows an example of a wireless SDMA TD/FD/CDMA system (Barratt et al., U.S. patent application Ser. No. 08/375,848) in which a number of subscriber stations (symbolically shown as handsets) 20, 22, 24 are being served by base station 100 that may be connected to a wide area network (WAN) 56 for providing any required data services and connections external to the immediate wireless system. Switching network 58 interfaces with WAN 56 for providing multichannel duplex operation with the WAN by switching incoming WAN data to lines 60 of base station 100 and switching outgoing signals from base station 100, on line 54 to the WAN. Incoming lines 60 are applied to signal modulators 62 that produce modulated signals 64 for each subscriber station the base station is transmitting to. A set of spatial multiplexing weights 74 for each subscriber station are applied to the respective modulated signals in spatial multiplexers 66 to produce spatially multiplexed signals 68 to be transmitted by a bank of multichannel transmitters 70 using transmit antenna array 18. The SDMA processor (SDMAP) 48 produces and maintains spatial signatures for each subscriber station for each conventional channel, calculates spatial multiplexing and demultiplexing weights for use by spatial multiplexers 66 and spatial demultiplexers 46, and uses the received signal measurements 44 to select a channel for a new connection. In this manner the signals from the current active subscriber stations, some of which may be active on the same conventional channel, are separated and interference and noise suppressed. When communicating from the base station to the subscriber stations, an optimized multilobe antenna radiation pattern tailored to the current active subscriber station connections and interference situation is created. An example of a transmit antenna pattern that may be created is shown in FIG. 2.
Returning to FIG. 1, spatial demultiplexers 46 combine received signal measurements 44 from the multichannel receivers 42 and associated antenna array 19 according to spatial demultiplexing weights 76, a separate set of demultiplexing weights being applied for each subscriber station communicating with the base station. The outputs of spatial demultiplexers 46 are spatially separated signals 50 for each subscriber station communicating with the base station, which are applied to signal demodulators 52 to produced demodulated received signals 54 for each subscriber station communicating with the base station. In an alternate embodiment, the demultiplexing and demodulation processing are performed together in a nonlinear multidimensional signal processing unit.
The demodulated received signals 54 are then available to switching network 58 and WAN 56.
In an FDMA system implementation, each multichannel receiver and each multichannel transmitter is capable of handling multiple frequency channels. In other embodiments, multichannel receivers 42 and multichannel transmitters 70 may instead handle multiple time slots, as in a TDMA system; multiple codes, as in a CDMA system, or some combination of these well known multiple access techniques (Barratt et al., U.S. patent application Ser. No. 08/375,848).
In practical systems that may involve hundreds or thousands of subscriber stations, perfect separation or orthogonality between every subscriber station, following the application of SDMA processing, cannot be insured because of the complexity and cost that such a requirement would place on the system design. If the separation of subscriber station connections post-SDMA processing cannot be ensured, the extended capacity of the SDMA will be limited and interference between subscribers will occur from the use of SDMA techniques. The consequence of this practical limitation is that a method for minimizing the interference and thereby maximizing the effective channel capacity of the SDMA system is required.
Even if two or more subscriber stations are not perfectly separated or orthogonal after SDMA processing, it still may be possible to share a common conventional channel in a TDMA, FDMA or CDMA system using SDMA technology. From a practical point of view, it is not required that the subscriber stations be perfectly separated after SDMA processing to share a common conventional channel. It is only required that the interference between subscribers sharing a common conventional channel post-SDMA processing be low enough so as not to reduce the quality of communications below a prescribed level.
Because of the interference introduced by frequency reuse and the fragile nature of orthogonality for conventional and spatial channels, all wireless multiple access communications systems need a method for base station and channel assignment that minimizes these adverse effects when a new call or connection between a base station and a subscriber is made. The labels new subscriber and new connection will be used interchangeably to denote a new call or connection between a base station and a subscriber station, and the labels active subscriber, existing connection and existing subscriber will be used interchangeably to denote a call or connection in-progress between a base station and a subscriber station. If not careful, the new subscriber may be assigned to a base station and a channel on which poor quality is experienced due to excessive interference. Moreover, the addition of a new subscriber has the potential consequence of adversely affecting the quality of communications on existing connections. Also, existing subscribers can suffer from increased channel interference from the addition of a new subscriber, or other unrelated causes, that can require moving subscribers from currently assigned channels to new channels in order to restore Acceptable quality communications. Channel re-assignment methods, using decision processes similar to those used for initial base station and channel assignment, are also required.
Prior art channel assignment and reassignment methods are based on measurements of physical phenomena such as the received signal strength indication (RSSI) or the co-channel interference on different conventional channels. Barnett, in U.S. Pat. No. 5,557,657, describes a method for handover between an overlay cell and an underlay cell depending on the RSSI. Booth (U.S. Pat. No. 5,555,445) describes a method for intercell handoff in which an intracell handoff from one conventional channel to another is first attempted, and the success or failure of the handoff is indicated by the RSSI. Knudsen (U.S. Pat. No. 5,448,621) describes a method for reallocating conventional channels between cells that depends on the number of unused conventional channels in each cell (i.e., the cell load). Grube et al. (U.S. Pat. No. 5,319,796) outlines a method for measuring co-channel interference on a conventional channel by placing additional receivers in the coverage area of the co-channel user and then transmitting feedback information on measured co-channel interference to a channel assignment controller. In all of these methods, the processes of channel assignment and reassignment do not take the spatial distribution of the subscribers into account, nor do they consider how the RSSI and co-channel interference jointly affect the signal quality of the new connection.
Hanabe (U.S. Pat. No. 5,475,864) describes a channel assignment method for sectorized cells which have static antenna beam patterns. Hanabe does not consider what happens with fully adaptive SDMA systems in which beam patterns dynamically change depending on which subscribers are active at any given time. Furthermore, the channel assignment of spatial channels made possible by SDMA is never addressed.
If two subscribers with similar spatial signatures were to be assigned to the same conventional channel, either at the same base station or at two different base stations, serious interference would render the channel unusable to both subscribers. Hence, there is a need for a new method of channel assignment for advanced, fully adaptive SDMA systems that can predict, a priori, the quality of a spatial or conventional channel; i.e., before the new connection is assigned to a given base station and channel. Also, there is a need for a SDMA channel assignment method that can predict the impact of a new connection on existing connections and can perform call admission control as necessary. The availability of such base station and channel assignment, reassignment, and admission control methods would allow SDMA methods to increase system capacity by better isolating subscribers while maintaining acceptable communications quality.